Good question Zaiku, I actually track this in my NSX Valuation Document linked in my signature in section 1e, by comparing the average strength of a sale at different venues as compared to the nominal value. Below is the current results, which includes both the $ paid by buyer, with buyer's fee; and the $ received by seller, without buyer's fee. The venues compared are Private sales, Auction (Barrett, Sotheby's, Mecum, etc), BaT, and eBay. I don't have access to any Manheim data, unfortunately.
<google-sheets-html-origin style="color: rgb(0, 0, 0); font-size: medium;">[TABLE="class: grid, width: 0"]
<tbody>[TR]
[TD][/TD]
[TD]Private[/TD]
[TD]Auction[/TD]
[TD]BaT[/TD]
[TD]eBay[/TD]
[/TR]
[TR]
[TD]$ Paid by Buyer (w/ Buyer's Fee) v Nominal Value[/TD]
[TD="bgcolor: #B6E2CD, align: center"]+3.7%[/TD]
[TD="bgcolor: #D1EDDF, align: center"]+2.9%[/TD]
[TD="bgcolor: #57BB8A, align: center"]+6.6%[/TD]
[TD="bgcolor: #FAE9E8, align: center"]~0.0%[/TD]
[/TR]
[TR]
[/TR]
[TR]
[/TR]
[TR]
[TD]$ Received by Seller (w/out Buyer's Fee)[/TD]
[TD="bgcolor: #B6E2CD, align: center"]+3.7%[/TD]
[TD="bgcolor: #E67C73, align: center"]-7.4%[/TD]
[TD="bgcolor: #FAE9E8, align: center"]~0.0%[/TD]
[TD="bgcolor: #FAE9E8, align: center"]~0.0%[/TD]
[/TR]
[TR]
[/TR]
[TR]
[/TR]
[TR]
[TD]# of NSXs Sold[/TD]
[TD="align: center"]135[/TD]
[TD="align: center"]51[/TD]
[TD="align: center"]83[/TD]
[TD="align: center"]163[/TD]
[/TR]
</tbody>[/TABLE]
</google-sheets-html-origin>
As you mention, you are right on the money about eBay tending to result in weaker sales than other venues; there is no statistical difference between and eBay sale and the nominal value.
Auctions tend to result in much lower $ received by the seller once the buyer's fee is removed from the sale price.
BaT shows a significant increase in $ paid by buyer when compared to the other venues, but the $ received by the seller is not statistically significantly different than the nominal value.
This NSX here is perhaps the most notable case of "auction drunk bidding", hammering for $99,000 after buyer's fee, 69.2% above its nominal value.
https://www.barrett-jackson.com/Events/Event/Details/2001-ACURA-NSX-T-VIN-00001-224618